The present invention relates to a process for the production of titanium-based alloy members by powder metallurgy.
Hitherto the processes used for producing titanium or titanium alloy members have involved the use either of direct casting or of fritting.
Processes involving direct casting have the disadvantage of requiring an additional low temperature forging stage to obtain the .alpha.+.beta. structure making it possible to give the members obtained a satisfactory resistance to cyclic fatigue.
Thus, it is known that titanium has an allotropic transformation at a temperature of 882.degree. C., so that the latter defines the stability region of two phases. The two phases are the .alpha. phase with a compact hexagonal structure which is stable below 882.degree. C. and the centred cubic .beta. phase which appears above 882.degree. C.
In the case of titanium alloy the presence of certain addition elements lead to a two-phase .alpha.+.beta. region which corresponds to a structure giving improved mechanical properties. However, to maintain this structure during the shaping operations it is necessary not to exceed the allotropic transformation temperature of the alloy, which varies as a function of the elements present in the latter. Most addition elements used in titanium alloys tend either to widen the existence region of the .alpha. phase or that of the .beta. phase. Moreover, certain elements such as aluminium are alphagenic elements which aid the formation of the .alpha. structure, whilst other elements such as vanadium, molybdenum, iron, chrome, manganese, niobium and copper are betagenic elements which aid the formation of the .beta. structure.
The processes for the production of titanium members using fritting generally consist of carrying out a hot isostatic fritting at a pressure of 1 to 1.5.multidot.10.sup.2 MPa for four hours. This takes place at a temperature of approximately 950.degree. C. when it is wished to maintain the .alpha. phase in the case of pure titanium or when it is wished to obtain the .alpha.+.beta. structure in the case of titanium alloys or at a temperature of approximately 1050.degree. C. on seeking the temperature range corresponding to the .beta. phase of pure titanium or of its alloys.
Such processes have the disadvantage of requiring high pressures and relatively long periods of time, which increases the cost of the members obtained.
When using powders of titanium or titanium alloy having a grain size above 100 .mu.m it is impossible to obtain a satisfactory densification of the powder at pressures below 1.multidot.10.sup.2 MPa, because the hot plasticity of the titanium is inadequate to obtain a satisfactory deformation of such powders.
However, it is possible to produce titanium or titanium alloy members by conventional fritting processes at pressures below 50 MPa and temperatures below 900.degree. C. when using kneaded and ground titanium or titanium alloy powders. However, in this case the members obtained are brittle due to a significant oxygen intergranular contamination.
U.S. Pat. No. 3,963,485 also discloses a process for producing titanium members by powder metallurgy in which a mixture of titanium powder and iron-coated titanium powder is used to improve the ductility of the members obtained.
However, this process is not suitable for obtaining a satisfactory densification, particularly in the case of difficultly deformable titanium alloy powders.
In addition, fritting processes do not make it possible to directly obtain members with a complex shape such as the discs of turbines having integrated blades and which specifically have a "ring" structure, i.e. a heterogenic structure characterized by the presence of large grains which are surrounded and welded together by finely crystallized grains.